A number of materials are known to exhibit a piezoelectric effect, being the generation of electricity under mechanical stress. Conversely, such materials may undergo a degree of physical deformation when subjected to an electrical field. These piezoelectric manifestations are utilised in a number of “piezoelectric” devices which generate either an electrical signal caused by pressure (used as sensors) or deformation by exposure to an electrical field (used as actuators). This piezoelectric effect is the result of the anisotropy of the crystalline structure (eg. Quartz, ferroelectric lead zirconate titanate (PZT) ceramics).
Ferroelectric ceramics, such as PZT, are now used in diverse technological applications where the unique piezoelectric (and ferroelectric) properties of these materials can be exploited. Piezoelectric devices have been made by forming PZT thin films on a substrate where the film thickness typically ranges from about 0.1 to 1.0 μm. These devices have use in microelectronic and micro-systems applications.
Piezoelectric devices have also been made of bulk piezoelectric ceramics. The bulk piezoelectric ceramics are formed by the sintering of discrete ceramic particles without a substrate.
There has, over recent years, been an increasing interest in piezoelectric ceramic thick film technology in relation to applications that combine device performance requirements approaching those of bulk piezoelectrics but with the miniature size scale required for use in microelectronic packages. The term “thick film” in the present context means a film having a thickness of between about 10 μm and 100 μm.
Thick layers of piezoelectric ceramic material are advantageous in that larger actuating forces, or (in the case of the reverse effect) larger sensing signals, may be generated (when compared to thin-film devices).
It is perceived that piezoelectric ceramic (eg PZT) thick films will have applications in various devices including chemical sensors, torque and pressure sensors, pyroelectric arrays, buzzers, micro-pumps, high frequency ultrasonic transducers, mass microbalances, microactuators, microtransducers, micro-accelerometers, micro-strain gauges and micro-resonators.
Various technologies are relatively well known for the formation of thin film piezoelectric ceramics and bulk ceramics. However, the integration of piezoelectric ceramic thick films into micro-electronic packages poses a number of technical difficulties relating primarily to processing and integration compatibility with the underlying substrate.
In recent years, it has been discovered that the deposition of piezoelectric ceramic powders (in paste form), by printing the paste onto the substrate, followed by annealing (ie firing), is an efficient method to form a piezoelectric thick film of about 10 μm to 100 μm on a substrate. This annealing step is done so as to sinter the particles together to form a cohesive film with suitable electrical and mechanical properties.
However, the processing temperature for sintering piezoelectric ceramic, such as PZT, is as high as 1,200° C. (for several hours). Temperatures of this magnitude are not compatible with many of the substrates, including electroded silicon. This is because at such elevated temperatures, lead diffuses into the bottom electrode and silicon substrate, and thus the substrate is damaged. Accordingly, in the formulation of piezoelectric ceramic thick films on silicon substrates, the maximum allowable firing (annealing) temperature is between about 900° C. and about 1,000° C.
Certain sintering aids for PZT and other piezoelectric ceramic thick films have been developed. Sintering aids for PZT which have been developed are generally low-melting metal oxide powders. The addition of these metal oxide powders relies, at least in part, on liquid-phase sintering for densification of the ceramic. The presence of such metal oxide powder enables sintering to occur at lower temperatures. The chosen metal oxide powder is typically added to the PZT powder through mechanical mixing and this is followed by the further addition of an organic carrier (or vehicle). The powders and the organic carrier are well mixed to form a paste. This paste is then, typically, screen-printed onto the substrate as a wet film. This wet film is then dried and the layered substrate then undergoes annealing at temperatures of between about 800° C. and 1,000° C. The above sintering aids (eg metal oxide powders) help to keep the annealing temperature at a level which inhibits (or reduces) diffusion of lead into the silicon substrate, and reduces the damage of the bottom electrode and the substrate.
However, the mechanical mixing method of the piezoelectric ceramic powder with the metal oxide powder, which is added as the sintering aid, results in a significant lack of homogeneity in the distribution of the metal oxide powder throughout the PZT powder in the as-deposited film. In turn, this lack of homogeneity results in a non-uniform distribution of the liquid-phase of the added sintering aids in the sintering process. This adversely affects the stability of the resultant thick film and the piezoelectric properties of the film. In comparison with bulk ceramic, this problem is particularly serious for the thick film in which a high sintering temperature and long sintering time are not allowed in order to minimise the damage of the bottom electrode and the substrate.
Accordingly, the present invention is directed towards a method of producing a piezoelectric thick film on a substrate with improved uniform distribution of the glass-bonding phase in the piezoelectric ceramic materials and, therefore, having more reliable piezoelectric performance.
Throughout this specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
Any discussion of documents, acts, materials, devices, articles or the like, which has been included in the present specification, is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.